Spectroscopic and Structural Investigations Reveal the Signaling Mechanism of a Luminescent Molybdate Sensor

A heteroditopic ligand H2-L consisting of a dihydroxybenzene (catechol)-unit linked via an amide bond to a pyridyl-unit and its methyl-protected precursor Me2-L were synthesized, characterized, and their photophysical properties investigated. The three accessible protonation states of the ligand, H3-L+, H2-L, and H-L−, showed distinct 1H NMR, absorption and emission spectroscopic characteristics that allow pH-sensing. The spectroscopic signatures obtained act as a guide to understand the signaling mechanism of the luminescent pH and molybdate sensor [Re(bpy)(CO)3(H2-L)]+. It was found that upon deprotonation of the 2-hydroxy group of H2-L, a ligand-based absorption band emerges that overlaps with the Re(dπ)→bpy metal-to-ligand charge transfer (MLCT) band of the sensor, reducing the quantum yield for emission on excitation in the 370 nm region. In addition, deprotonation of the catechol-unit leads to quenching of the emission from the Re(dπ)→bpy 3MLCT state, consistent with photoinduced electron transfer from the electron-rich, deprotonated catecholate to the Re-based luminophore. Finally, reaction of 2 equiv of [Re(bpy)(CO)3(H2-L)]+ with molybdate was shown to give the zwitterionic Mo(VI) complex [MoO2{Re(CO)3(bpy)(L)}2], as confirmed by electrospray ionization (ESI) mass spectrometry and X-ray crystallography. The crystal structure determination revealed that two fully deprotonated sensor molecules are bound via their oxygen-donors to a cis-dioxo-MoO2 center.